This invention relates to electrical apparatus of the type including a power source, means operated by the power source, and means operatively connected between the power source and the means operated thereby for applying power of a predetermined magnitude to said operated means for operating same.
Apparatus of the type described is subject to cycling. That is to say, under certain operating conditions the operated device will become inoperative for an interval during which the conditions are alleviated so that the operating device is again operative. Cycling can present a severe maintenance problem since, among other things, the power applying means can deteriorate or burn out if the cycling persists.
Apparatus particularly prone to the adverse effects of cycling includes that for operating high pressure sodium lamps. This apparatus includes the lamp and a ballast-starter arrangement connected to a source of electrical power and operative for supplying voltage to the lamp to operate the latter. The operating characteristics of the sodium lamp are such that as the lamp ages some of its electrode material deposits on the arc tube of the lamp. This causes the arc tube to retain heat and, in turn, the internal lamp pressure and the arc tube voltage will increase. When the arc tube voltage becomes so high that the ballast can no longer support the arc tube, the lamp goes out. The lamp will re-light after it has cooled down sufficiently, and hence alternately cycles between lighted and extinguished conditions.
Accordingly, in order to avoid maintenance problems occurring because of a deterioration or burn-out of the ballast-starter arrangement due to cycling it is desirable, if not necessary to turn off power to the lamp when the lamp cycles more than a selected number of times.
Prior art devices attempt to accomplish this in several ways, for example, the voltage across the lamp or across the ballast-starter arrangement may be measured and power to the lamp may be monitored on the basis of this measurement. Also, monitoring circuitry can be included to determine if the ballast-starter arrangement is operating properly.
The present invention is aware of U.S. Pat. No. 4,207,500 issued on June 10, 1980 to George Duve, et al and assigned to Area Lighting Research Inc., Hackettstown, N.J.; U.S. Pat. No. 4,473,779 issued on Sept. 25, 1984 to Larry A. Lindner, et al and assigned to the same assignee; and U.S. Pat. No. 4,665,346 issued on May 12, 1987 to Pierre Tarroux and assigned to Europhane, Paris, France.
The Duve patent teaches an arrangement including sensor means for detecting the magnitude of the voltage across the extinguished lamp, and for generating an electrical current signal when the magnitude of the voltage across the lamp reaches a threshold magnitude which is greater than a predetermined magnitude. Means are responsive to the current signal for disconnecting the power source from the ballast-starter arrangement to thereby protect the latter from damage in the event of a cycling malfunction.
The Lindner patent recognizes that the power factor of an extinguished lamp-ballast-starter combination during cycling is different than that when the lamp is operative. Hence, the Lindner invention teaches sensor means for detecting the power factor across the lamp-ballast-starter combination and for generating a cut-off signal when the detected power factor is different from a predetermined power factor for disconnecting the power source from the ballast-starter arrangement to likewise protect the latter in the event of excessive cycling.
The Tarroux patent teaches operating a starting circuit for a predetermined interval in response to a control signal which is generated either following the application of a voltage to the input terminals of a lamp circuit at the beginning of a working period of said circuit, or a pre-determined delay following interruption of said voltage due to a short power interruption. The operation of the starting circuit is inhibited if the lamp extinguishes due to an increase in its operating voltage. Thus, attempts to re-start a defective lamp are avoided.
The present invention differs from the prior art in that a resistor with very low resistance value is included in the lamp-ballast circuit. The voltage across the resistor drops when the lamp changes from a lighted to an extinguished condition, i.e. when the lamp cycles. This change in voltage level is indicated by a comparator as a cycle which is counted and power to the lamp circuit is interrupted after a predetermined number of cycles occur. Thus, the cycling phenomenon is limited to protect deterioration of the ballast-starter arrangement as is desirable.